Bed bug bites: how to recognise them (and why they aren't enough)

What they look like

The typical lesion is a 2–5 mm itchy papule with a central puncture mark — sometimes haemorrhagic — ringed by redness. In more reactive people it can grow into far larger weals, up to five centimetres across, with intense itching.

Distribution matters more than shape: bed bugs bite exposed skin — face, neck, arms, legs. That is the opposite of scabies, which favours the finger webs and the sacral area, or body lice, which stay under clothing.

The "breakfast, lunch and dinner" sign

It's the detail everyone knows: three bites in a line or a triangle, a few centimetres apart. It is real, and it is described in the dermatological literature.

The problem is what people infer from it. The paper that made it famous is titled, literally, "a hallmark of flea and bedbug bites": that pattern is as characteristic of infestation by fleas — human, canine and feline — as by bed bugs. Finding three bites in a row does not tell you which of the two you have. And the two problems are solved in completely different places.

The usual explanation is wrong too. It isn't "one bug moving between courses": the insect tends to feed in one spot until it is full. The pattern comes rather from the parasite marking the most favourable patch of skin with salivary apyrase, and from the fact that if the meal is interrupted — because you moved in your sleep — it immediately finds a nearby spot. The three bites aren't a menu: they're one meal disturbed three times.

The real trap: the sensitisation ladder

This is what makes bites useless as an alarm, and it's the best-evidenced point in the article.

The reaction to a bite is not a property of the insect: it is an immune response that has to be built. People never exposed before typically don't react at all. In a prospective study on volunteers, 54% had no reaction to their first bites. Then the body learns:

• First exposure — no visible reaction in most cases.
• Once sensitised — symptoms appear 6–11 days after the bite.
• With further exposures — down to 2–3 days.
• Then only hours, until it becomes near-immediate.

Read that ladder backwards and here is what it means: while the infestation is small and cheap to fix, it leaves no marks. By the time the marks show up promptly the next morning, the biting has been going on for weeks. Bites aren't an early warning: they're the receipt.

And some people never react

Sensitisation does arrive eventually for almost everyone: in that same study, 18 volunteers out of 19 ended up developing a reaction. But in real populations the picture changes, and it changes exactly where it matters.

In a survey of 474 people living in confirmed infestations, 42% of the over-65s reported no reaction at all. Not "a mild reaction": none. Which means that in a care home, a dormitory or public housing, waiting for someone to complain about bites is a surveillance strategy that starts out already lost.

A similar figure circulates for children (41% non-reactors), but it deserves honest handling: the source itself notes the sample size was not statistically significant. We cite it for completeness, not as proof.

What they're confused with

The dermatological atlas devoted to the subject lists a long set of mimics. The trickiest ones:

• Fleas — same linear pattern, lesions up to 5–10 mm. The commonest confusion, and the most consequential: it changes where you intervene entirely.
• Scabies — 3–5 mm, but it seeks the finger webs and the sacral area, not exposed skin.
• Urticaria — the lesions move within 48 hours; bites stay where they are.
• Bullous pemphigoid — affects the elderly, is symmetrical and can involve the mucosa. In a care home this is precisely the mistake that can be made in both directions.
• Shingles — grouped vesicles, unilateral, following a dermatome.
• Spider, tick, bee and wasp — usually a single lesion, and with bees and wasps the reaction is immediate.

One detail settles the matter: there is no specific sign of a bed bug bite on histology. Not even a biopsy tells them apart with certainty. It goes without saying that a diagnosis is a doctor's job, not a web page's — least of all this one's.

So are bites any use?

Here the sources don't agree, and it's worth reporting that rather than picking the one that suits us.

The dermatological atlas is optimistic: "clinical appearance with the often linear orientation and medical history are usually enough to make a proper diagnosis". The standard clinical review is stricter: "ultimately, a positive insect identification is the only sure way of definitively diagnosing the cause".

It isn't a contradiction: they answer different questions. The dermatologist has a person who reacts in front of them and must work out what bit them — and there, pattern, history and distribution are usually enough. Whoever runs a building has a different problem: they need to know whether twenty rooms hold a colony. For that question, people's skin is a terrible instrument, because half of them don't react at first and 42% of the elderly never react at all.

Bites describe a person. They don't describe a building.

What to look at instead

Both sources agree on what actually confirms it, and it isn't skin: the traces and the insect. Digested-blood spotting on the linen and in the mattress seams, cast skins, eggs, and the bug itself. They're the signs we set out on the bed bug page, and they're the only thing that answers the right question.

It's also why our work exists. The dog doesn't look for bites: it looks for the insect, which is the only evidence that counts — and it looks for it in cracks a few millimetres wide, before anyone has had time to become sensitised.